The present invention relates to a thermal printing system, and particularly, to a color thermal transfer printer.
A variety of color thermal transfer printing system have been proposed in which a color ink or other record-forming material is selectively transferred from a carrier sheet, such as an ink ribbon, to a record medium (e.g., plain paper or the like) by applying thermal energy to localized areas on the carrier. For example, U.S. Pat. No. 4,250,511 describes a system (FIG. 2) which includes a printhead having a plurality of individual thermal elements that are selectively energized in a timed relationship to transfer a mosaic arrangement of inks arranged on the carrier sheet to a paper as it passes over the printhead. The inks on the carrier include four distinct and different repeating patterns of colors (i.e., three primary colors and black) permitting the formation of multicolor images on the record medium. The images are formed by energizing the appropriate thermal elements as the desired color inks are moved into registration with the elements.
This system, however, has several practical problems. First, due to the use of a mosaic color pattern, circuitry for activating a particular color is very complicated and costly since synchronization is needed, based upon the position of a particular mosaic in two directions for achieving accurate registration. Second, such mosaics are difficult and costly to manufacture. In fact, if high resolution is desired the mosaic color dot spacing must be decreased which increases manufacturing difficulties and cost. Further, the higher resolution desired complicates the synchronization problems discussed above. Third, such mosaics do not permit superimposing of color patterns for creating greater color hues and color ranges.
U.S. Pat. No. 4,250,511 also shows a system (FIG. 4) wherein each discrete respective color area extends transversely along the width of the carrier sheet and the areas are alternately and successively arranged along the length of the carrier. Likewise, this system has several practical problems. First, the carrier sheet moves faster than the record medium; consequently good ink transfer does not occur and smearing is produced resulting in poor resolution. Second, to reduce smearing a smaller transfer force must be provided between the printing head and the record medium; as a result, complete ink transfer is not provided due to inadequate heating. In fact, the transferred ink can frequency bleed off to other surfaces upon its subsequent contact with the record medium.
Another system, shown in U.S. Pat. No. 4,067,017, discloses another system whereby color images are imparted to a rapidly advancing record medium from separate red, green and blue carrier ribbons under the control of three separate elongated assemblages of thermal elements extending across the paper, each corresponding to a respective carrier ribbon. As shown in FIG. 4 of that patent, the ribbons move in a direction transverse to the direction of movement of the record medium. This system, however, has the disadvantage that it is difficult to fully and completely transfer the ink from the ribbons onto the record medium since the paper moves relative to the ribbons. As a result, blurring or smearing of the ink will occur. Another disadvantage is that complicated synchronization is needed based upon the movement of three separate ribbons in one direction and the movement of the paper in another direction. Further, superimposing color patterns for creating greater color hues and color images is complicated.